1. Technical Field
This invention relates generally to multilayer circuit substrates, and more particularly to conductive vias that facilitate signal propagation between intermediate layers within circuit substrates.
2. Description of the Background Art
A typical circuit substrate includes multiple conductive layers separated by electrically insulated layers. Such conductive layers intercommunicate using vias formed in the circuit substrate. Generally, vias are perpendicular bores formed through the layers of a circuit substrate by processes such as laser drilling. Such bores are filled or lined with conductive material as needed to provide electrical communication paths between the conductive layers. Vias typically pass through the entire circuit substrate, even if for example the top layer is communicating with a middle layer. Such vias are commonly known as “through-hole vias.”
As data-communication speeds increase, signal integrity becomes crucial for successful data transmission. Due to the increasing signal density on circuit substrates, an increasing number of signal layers becomes unavoidable. Consequently, an increasing number of vias is needed to route signals between the conductive layers. However, at high data-communication speeds, through-hole vias may cause signal degradation.
FIG. 1 shows a cross-sectional side view of a prior art multilayer PCB 100, which includes a substrate 102, a data input line 104, a data output line 106, a via 108, and multiple ground planes 110. Note that via 108 is continuous through PCB 100. Data input line 104 routes signals to data output line 106 through an upper via portion 112 of via 108. Because only upper portion 112 of via 108 is used to facilitate signal propagation between data input line 104 and data output line 106, the unused portion of via 108 defines an open-ended stub 114.
Although through-hole vias 108 do not add significant cost to the manufacturing process, they have substantial disadvantages. Open-ended stubs 114 can cause signal degradation, jitter and eye diagram closure. For example, when an electrical signal propagates through data input line 104, the signal reaches via 108 and propagates through upper via portion 112 until a point 116 where data output line 106, upper via portion 112, and open-ended stub 114 meet. At point 116, a component of the signal propagates through data output line 106, while another component of the signal propagates through open-ended stub 114. The signal propagating through stub 114 reflects back and interferes with the signal propagating from data input line 104. Further, such open-ended stubs 114 create excess capacitance and inductance, further degrading signal integrity. Excess inductance and capacitance is another way of explaining the same phenomena of reflecting energy from an open stub. Both views are correct. When the open stub is modeled as lumped elements, then we can speak of inductance and capacitance. When the model is done with transmission lines, then one can describe as propagating signals on transmission lines with certain characteristic impedances.
FIG. 2 shows a circuit 200 corresponding to prior art multilayer PCB 100. By modeling the elements as transmission lines, one skilled in the art will easily see the negative effects of the open-ended stub 114. In FIG. 2, data input line 104 and data output line 106 of FIG. 1 are represented as transmission line element 202 and transmission line element 206, respectively. Open-ended stub 114 is represented as transmission line element 208. Because transmission line element 208 is open ended, one skilled in the art will recognize that reflections will cause signal degradation of the signal traveling from transmission line element 202 to transmission line element 208.
FIG. 3 shows a cross-sectional side view of a multilayer PCB 300 that provides a prior art solution to alleviate signal degradation caused by open-ended stubs, e.g., open-ended stub 114 of FIG. 1. Multilayer PCB 300 includes a substrate 302, a data input line 304, a data output line 306, a blind via 308, and multiple ground planes 310. As shown, blind via 308 is not continuous through PCB 300. It extends from data input line 304 only to data output line 306. Thus, there is no open-ended stub to degrade the signal. Typically, blind vias such as blind via 308 are formed by control-depth drilling (CDD) techniques known to those skilled in the art. For example, a laser drill may be used to form a bore a controlled distance through the circuit substrate. The bore may then be filled or lined with conductive material (e.g., copper) as needed. Although blind vias reduce signal degradation, the manufacturing process adds substantial cost compared to that of typical through-hole vias. Further, in the case of backplane connectors (through-hole pins), the blind via process is useless.
What are needed are less expensive systems and methods for intercommunicating signals in a multilayer circuit substrate without or with reduced signal degradation.